Kinases or phosphotransferases are enzymes which transfer phosphate groups from high-energy donor molecules such as adenosine triphosphate (ATP) to specific substrates in a process known as phosphorylation. The chemical activity of a protein kinase involves transferring a phosphate group from a nucleoside triphosphate such as ATP and covalently attaching it to one of three amino acids that have a free hydroxyl group. Most protein kinases act on both serine and threonine, others act on tyrosine and some act on all three of these amino acids. There are also protein kinases that phosphorylate other amino acids, including histidine.
Protein kinases play a fundamental role in signal transduction and thus are key mediators of essential cellular processes such as metabolism, growth, cell cycle progression, migration and apoptosis. Disruption of signalling pathways is associated with the pathology of many diseases, including cancer, and thus understanding the activity of the enzymes that control transduction is of great interest clue to their potential as therapeutic targets and biomarkers.
Protein phosphorylation is a reflection of kinase activity and current mass spectrometry (MS) based phosphoproteomic analysis provides the unprecedented opportunity to evaluate the activity of multiple signalling pathways in a single assay. However, such global approaches are not yet possible because in most cases we do not yet know the identity of the kinases acting on the sites that we can measure using phosphoproteomic techniques. Furthermore, there are more than 500 kinase genes in the human genome and despite intense research, methods for global and unbiased analysis of kinase activities have not yet been reported.